Variable rate load setback circuit

ABSTRACT

A turbine control circuit which adjust the rate of power reduction by adjusting a load reference signal to control a steam control valve to compensate for the rate of loss of steam for the continued operation of a turbine with a failed component in the steam generating loop.

VARIABLE RATE LOAD SETBACK CIRCUIT BACKGROUND OF THE INVENTION Thepresent invention relates to control circuitry for a turbine system andis especially applicable to control of steam turbine systems.

In operating steam turbine systems, it is important to reduce themechanical power output of the turbine gradually rather than an abruptlyduring a failure of some critical components in the power plant, such asforced draft fans, coal crushers, boiler feed-water pumps or the like.When such a failure occurs, it is mandatory to reduce the steam flowand, consequently, the mechanical power output of the turbine. The rateof reduction of steam flow to the turbine should bear some relationshipto the criticality of the failed component, the amount of load reductionnecessary for continued operation without the failed component, andwhether or not several components failed simultaneously orconsecutively.

Present load setback circuits, as evidenced by U.S. Pat. No. 3,561,216,issued upon an application by J. H. Moore, Jr., and U.S. Pat. No.3,340,883, issued upon an application by J. R. Peternel, both of whichpatents are assigned to the present assignee, accomplish the task ofproducing a load reference signal to control the steam control valve toreduce the steam flow in proportion to the criticality of the failedcomponent and number of failures. However, present setback circuits lackmeans of adjusting the steam control valve to compensate for the rate ofthe loss of steam.

This problem has been substantially eliminated by I providing in apreferred embodiment of my invention a novel variable rate load setbackcircuit which has the ability to reduce the steam flow to any desiredlevel, as determined by a plant operator. When compensation for the rateof the loss of steam is desired, a switch will be activated which willset a preselected bias to an input of a first operational amplifier inan integrator circuit to decrease the selected load reference signal ata predetermined rate to form a load setback reference signal which willcontrol the steam control valve. Simultaneously, a preset bias to aninput of a second operational amplifier in a limiter is applied and thisaction clamps the load setback reference signal at the preset level.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide a new and improved variable rate load setback circuit which willautomatically adjust the rate of power reduction to compensate for therate of loss of steam in a turbine system by controlling the steamcontrol valve with a load setback reference signal.

It is another object of this invention to provide a new load setbackcircuit in which a plant operator can alter at will, the flow rate ofsteam.

It is a further object of this invention't'o provide a new and improvedvariable load setback circuit for a steam turbine which responds at thefastest desired rate and to the lowest desired level automatically.

Briefly stated and according to one aspect of the invention, theforegoing objects are achieved by producing a new and improved variablerateload setback circuit comprising an integrator and a limiter. When anamplifier in the integrator and an amplifier in the limiter areindividually biased at a value determined by the operator, the loadreference signal will be decreased from its operating value at apredetermined rate and clamped at a predetermined level to form a loadsetback reference signal. The load setback reference signal provides ameans for the adjustment of the rate of power reduction by controllingthe steam control valve and thus compensate for the rate of loss ofsteam in a turbine system with a failed component in the steamgenerating loop.

BRIEF DESCRIPTION OF THE DRAWING The invention, both as to itsorganization and principle of operation together with further objectsand advantages thereof may better be understood by reference to thefollowing detailed description of an embodiment of the invention whentaken in conjunction with the accompanying drawing in which the soleFIG- URE is a circuit diagram illustrating the basic components of avariable rate load setback circuit in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, inputterminal 11 provides a connection to receive a load reference signal orvoltage from an amplifier which in the prior art is used to control asteam control valve. The load on the turbine is proportional to thisload reference signal. The input terminal 11 is connected through diode12 to a point A. The voltage at point A is held at a value correspondingto the load reference signal; Point A with respect to ground is also theoutput of the variable rate load setback circuit and this outputultimately controls a steam control valve (not shown). Resistors 13 and14 are impressed with a voltage to provide a positive biasing voltage todiode 12.

Diode 15, which also receives a positive bias through resistors 13 and14, is shown connected in a first feedback loop of integrator 17 and ina second feedback loop of limiter 18. A current amplifier 16 comprisesresistors 19 and 20, which produce voltage drops to allow a transistor21 to perform its normal current amplification function, and areconnected to the collecter and emitter respectively of transistor 21.The emitter of transistor 21 is also connected to point A through diode15. Note that diode 15 forms an overall low value gate between theintegrator 17, limiter l8, and the load reference signal applied atinput terminal 11.

Integrator 17, which will provide a linearly decreasing voltage (fromthe load reference signal) at pointA, is determined by an operatormandated setting of a control. Integrator 17 is connected in a feedbackloop from'the output of an operational amplifier 23 through a diode.24to the base of transistor 21 in currentamplifier 1'6 and seriallythrough the emitter of transistor 21 and diode 15 'to the input ofintegrator 17 through capacitor 22. Input resistors 25 and 26 form avoltagedividerand along with.normally closed relaycontact 27 apply afixed positivebiasito operational amplifier 23 to keep operationalamplifier 23 in negative'saturation. Also connected at an input tooperational amplifier'23 is resistor 28, the other end of which resistoris connected to ground. Resistor 28 is utilized'toreducethe inputimpedance of operational amplifier 23 to overcome the extremely highimpedance of amplifier 23 which would cause capacitor 22 to slowlycontinue chargingand'thus produce large steps whenever the integrator 17was operating. Resistor 29 is utilized to control the amount of inputvoltage, as a preset bias, to be applied to an input of operationalamplifier 23 through an input resistor 31 when normally open contact 30is closed.

Limiter 18, which clamps the load reference setback signal at a presetvalue controlled by the operator, is connected to point A in a secondfeedback loop through resistor 32 to an input of operational amplifier33. The output of operational amplifier 33 is applied to the base oftransistor 21 in current amplifier 16 through a high value gate or diode34 and from the emitter of transistor 21 through diode 15 to point A.Resistor 35 applies a negative voltage through input resistor 36 to aninput of operational amplifier 33 to maintain operational amplifier 33in positive saturation. Resistor 37 controls the voltage or preset biasapplied to an input of operational amplifier 33 through closure ofcontact 38, which is normally open, and serially connected inputresistor 39.

When it is desired to activate the variable rate load setback circuit,normally closed contact 27 is opened and normally open contact 30 inintegrator 17 and contact 38 in limiter 18 are set, or closed. This isdone in a manner well known in the art, such as energizing a coil of arelay which in turn closes contacts 30 and 38 and opens contact 27.

This drawing, for the sake of simplicity, shows only one input terminal1 1 from the turbine system and only one variable rate load setbackinput. However, several of these inputs in practice are connected inparallel in a manner well known in the art, and the smallest signalpresent from any output ultimately controls the load signal.

In operation, the voltage at point A is initially held at whatever valuecorresponds with the load reference signal applied to input terminal 11.

Before the operator activates the setback circuit, the operationalamplifier 23 in integrator 17 has a positive bias applied to its inputto keep amplifier 23 saturated negatively, and the capacitor 22 chargesto the voltage value at point A. The positive bias applied to amplifier23 allows for a very fast integration rate. Therefore, the capacitor canrapidly follow changes in the voltage at point A.

In limiter 18, the negative bias applied through resistors 35 and 36 tooperational amplifier 33 keeps amplifier 33 in positive saturationbecause of the low value gate diode l blocking feedback voltage in thesecond feedback loop of limiter 18. The voltage value of the negativebias applied to operational amplifier 33 is kept at a small incrementabove the largest positive voltage at point A, ever to be encountered inorder to prevent diode from conducting. The conduction of diode 15 isprevented due to the polarity reversal of operational amplifier 33inherent in such an amplifier.

When automatically operated relay contacts 30 and 38 close and contact27 opens, the load setback is initiated. The opening of contact 27removes the fast integration rate from integrator 17 and, in closing,contact 30 applies a positive preset rate voltage to operationalamplifier 33. Simultaneously, the closing of contact 38 applies a presetpositive voltage to limiter 18. This new preset voltage in limiter 18 ispositive but smaller in absolute value than the normal negative biasvoltage of amplifier 33 and thus produces a net voltage subtraction atthe input of operational amplifier 23 and thus a reduced positivevoltage at the anode of diode 34.

The reduction of the anode voltage of diode 34 to a smaller positivevalue than present at point A biases diode 15 on, and causes the voltageat point A to begin to fall. This voltage drop is felt by capacitor 22in integrator l7, and capacitor 22 begins to discharge into the input ofoperational amplifier 23. This discharge changes the input tooperational amplifier 23 in a negative direction, driving amplifier 23towards positive saturation. Amplifier 23 does not reach positivesaturation in that, when the anode of diode 24 reaches the value ofvoltage slightly above that of its cathode, diode 24 conducts limitingthe rise to the voltage value existing at point A. Integrator 17 takescontrol of the circuit operation and forces the output voltage at pointA to de crease at the setback rate determined by the ohmic value ofresistor 31, the preset value of voltage at resistor 29, and thecapacitive value of capacitor 22.

When the voltage at the anode of diode 34 in limiter l8 falls due to theclosing of contact 38, the reduction of voltage, as pointed outpreviously is initiated in that the limiter 18 is low value gated withrespect to point A by diode 15. It is also high value gated with respectto amplifier 23 by diode 24. When diode 24 conducts as described above,operational amplifier 33 is forced into negative saturation due to thepositive feedback voltage at the input of amplifier 33 through resistor32. This voltage reduces as the voltage at point A reduces.

When the voltage output of operational amplifier 23 has been reduced atthe preset rate to a smaller positive value than the difference betweenthe bias at resistor 35 and at resistor 37, the limiter 18 due to itsbeing high value gated with operational amplifier 23, and since thevoltage at the input of operational amplifier 23 has already reachedeffective zero, will come out of negative saturation and take control.The voltage at point A will stabilize at the preset limit whileoeprational amplifier 23 will be forced back into negative saturationfrom loss of feedback through diode 15 and the reversion of theeffective voltage at the input of operational amplifier 23 to a positivevalue.

To remove the effect of the variable rate load setback circuit, normallyopen contacts 30 and 38 are reopened, and normally closed contact 27 isreclosed. The negative bias on operational amplifier 33, now free of thepositive preset voltage from resistor-37, forces the anode of diode 34to increase in a positive polarity direction. The effect of positivevoltage placed at the input of operational amplifier 23 by the closureof contact 27 plus the increase in voltage at point A rapidly chargesthe capacitor 22 and forces operational amplifier 23 in integrator 17into negative saturation. As soon as operational amplifier 33 losescontrol to the load reference signal, the loss of feedback forces itinto positive saturation and the circuit is again ready for the nextdemand for a load setback.

It has been shown that, by providing an integrator which will produce alinearly decreasing voltage from a load reference signal and a limiterwhich will clamp the load reference signal at a preset value, a normalload reference signal can be compensated for the rate of the loss ofsteam to ultimately adjust the rate of power reduction by controlling asteam control valve.

While an embodiment and application of this invention has been shown anddescribed, it will be apparent to those skilled in the art that manymore modifications are possible without departing from the inventiveconcepts herein described. The invention, therefore, is not to berestricted except as is necessary by the prior art and by the spirit ofthe appended claims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A turbine control circuit comprising:

means for receiving a load reference signal proportional to a desiredload on a turbine;

means for linearly decreasing the load reference signal at apredetermined rate to form a load setback reference signal; and

means for clamping the load setback reference signal at a preset level.

2. A turbine control circuit as in claim 1 wherein said means forlinearly decreasing the load reference signal is an integrator.

3. A turbine control circuit as in claim 2 wherein said integratorfurther comprises a first operational amplifier having a first feedbackloop, a capacitor in series in said first feedback loop, and means forconnecting a first preset bias to an input of said first operationalamplifier.

4. A turbine control circuit as in claim 3 wherein said first presetbias is variable.

5. A turbine control circuit as in claim 1 wherein said means forclamping the load setback reference signal is a limiter.

6. A turbine control circuit as in claim 5 wherein said limiter furthercomprises a second operational amplifier having a second feedback loop,a resistor in series in said second feedback loop, and means forconnecting a second preset bias to an input of said second operationalamplifier.

7. A turbine control circuit as in claim 6 wherein said second presetbias is variable.

8. A turbine control circuit as in claim 3 wherein said means forconnecting the first preset bias is remotely controlled.

9. A turbine control circuit as in claim 6 wherein said means forconnecting the second preset bias is remotely controlled.

10. A turbine control circuit comprising:

means for receiving a load reference signal proportional to a desiredload on the turbine;

an integrator circuit for linearly decreasing the load reference signalat a predetermined rate to form a load setback reference signal, saidintegrator circuit further comprising a first operational amplifierhaving a first feedback loop, a capacitor in series in said firstfeedback loop, and means for connecting a first preset bias to an inputof said first operational amplifier; and

a limiter circuit for clamping said load setback reference signal at apreset level comprising a second operational amplifier having a secondfeedback loop and means for connecting a second preset bias to an inputof said second operational amplifier.

11. A turbine control circuit as in claim 10 wherein said first andsecond preset biases are variable.

12. A turbine control circuit as in claim 10 wherein said means forconnecting the first preset bias and said means for connecting thesecond preset bias are simultaneously remotely controlled.

1. A turbine control circuit comprising: means for receiving a loadreference signal proportional to a desired load on a turbine; means forlinearly decreasing the load reference signal at a predetermined rate toform a load setback reference signal; and means for clamping the loadsetback reference signal at a preset level.
 2. A turbine control circuitas in claim 1 wherein said means for linearly decreasing the loadreference signal is an integrator.
 3. A turbine control circuit as inclaim 2 wherein said integrator further comprises a first operationalamplifier having a first feedback loop, a capacitor in series in saidfirst feedback loop, and means for connecting a first preset bias to aninput of said first operational amplifier.
 4. A turbine control circuitas in claim 3 wherein said first preset bias is variable.
 5. A turbinecontrol circuit as in claim 1 wherein said means for clamping the loadsetback reference signal is a limiter.
 6. A turbine contrOl circuit asin claim 5 wherein said limiter further comprises a second operationalamplifier having a second feedback loop, a resistor in series in saidsecond feedback loop, and means for connecting a second preset bias toan input of said second operational amplifier.
 7. A turbine controlcircuit as in claim 6 wherein said second preset bias is variable.
 8. Aturbine control circuit as in claim 3 wherein said means for connectingthe first preset bias is remotely controlled.
 9. A turbine controlcircuit as in claim 6 wherein said means for connecting the secondpreset bias is remotely controlled.
 10. A turbine control circuitcomprising: means for receiving a load reference signal proportional toa desired load on the turbine; an integrator circuit for linearlydecreasing the load reference signal at a predetermined rate to form aload setback reference signal, said integrator circuit furthercomprising a first operational amplifier having a first feedback loop, acapacitor in series in said first feedback loop, and means forconnecting a first preset bias to an input of said first operationalamplifier; and a limiter circuit for clamping said load setbackreference signal at a preset level comprising a second operationalamplifier having a second feedback loop and means for connecting asecond preset bias to an input of said second operational amplifier. 11.A turbine control circuit as in claim 10 wherein said first and secondpreset biases are variable.
 12. A turbine control circuit as in claim 10wherein said means for connecting the first preset bias and said meansfor connecting the second preset bias are simultaneously remotelycontrolled.